In the art of casting molten metals, the metals are first reduced to a molten state at an elevated temperature by any one of a variety of different types of furnaces. The molten state (fluid state) is produced by a furnace which may be, for examples, a fuel gas fired furnace or an electric induction furnace. The molten metal in a fluid state is flowed generally from the furnace either directly into the mold for the casting or into some sort of a secondary receiver such as, for example, a holding furnace which maintains the metal in a molten state and serves to act as an intermediary in the transport of the molten metal to the casting molds.
One of the major detriments that can occur during the casting process is for impurities, in the form of solids, to be included in the molten metal as it is finally poured into the casting molds. The inclusion of such impurities into the castings, in many cases, is cause for the castings to be rejected and thus of no commercial value except for scrap.
There are four general types of inclusions which find their way into castings. These are (1) ceramic refractories from crucibles which serve as transports for the molten metal and as the lining for the melting furnaces, (2) slags which form on or near the surface of the molten metal in the melting furnace itself, (3) oxides and nitrides from atmospheric reactions of the molten metal constituents with the ambient atmosphere both in the melting furnace and during the course of its transport to the casting molds, and (4) residual oxidation reaction products which form from the oxidation of the alloying elements (which are included in the molten metal), such as silicon, magnesium, chromium, titanium, aluminum, yttrium, tantalum, columbium, zirconium, lithium and hafnium. These oxidation reaction products are often picked up from the refractory linings within the melting furnace and the holding furnace. In addition to the foregoing, in some instances, the molten metal fluids tend to entrap dissolved gases, such as hydrogen, oxygen and nitrogen therein. During solidification of the molten metal, the gases precipitate out of solution and form gas bubbles or pockets which are also considered to be detrimental in effect to the finished metal casting.
It is known in the art of metal casting that impurities and inclusions can be removed from molten metals generally, and in particular from molten aluminum, by flowing the molten metal fluid materials through a porous refractory or ceramic filter medium to produce filtration. Reichmann U.S. Pat. No. 2,021,520 discloses the use of a porous plate, formed from the grains of fired refractory metallic oxides combined with finely divided calcined particles of metallic oxides, for performing the filtration of molten aluminum. Burroughs U.S. Pat. No. 3,235,089 discloses a bonded ceramic filter medium and the process for making it from an essentially alumina composite mixture with fluxes. McDonald et al. U.S. Pat. No. 3,524,548 discloses a porous ceramic molten metal filter of a defined composition which is to be variously shaped into either flat plates or cylindrical closed-end tubes which depend downwardly from a horizontal sealing plate. The closed-end tubes disclosed in this item of prior art are cemented to the lower side of the base plate. The cement is depended upon both for sealing and for structural support. A variation of the McDonald et al theme is shown in Nowak U.S. Pat. No. 3,747,765. Nowak discusses the problems inherent in the McDonald et al design, and the problems caused by failure of the cement. Nowak seeks to cure both the problem of the cement and the fragility of the porous ceramic structure by using metal sealing plates with a draw-bolt holding the structure together. Like McDonald et al, Nowak discloses an arrangement wherein the cylindrical closed-end tubes depend downwardly from a horizontal sealing plate.
Pryor et al. U.S. Pat. No. 3,893,917 discloses the use of an open cell ceramic foam as a molten metal filter. Pryor et al. U.S. Pat. No. 3,947,363 discloses a method of producing an open cell ceramic foam filter. Yarwood et al U.S. Pat. No. 4,024,056 discloses the adaptation of open cell ceramic foam in a plate form to the filtering of molten metal. Pryor et al. U.S. Pat. No. 4,056,586 also discloses a method for producing an open celled ceramic foam filter medium. And finally, Brockmeyer U.S. Pat. No. 4,343,704 discloses yet another method and material for producing an open celled ceramic foam filter for molten metals.
All of the foregoing types of filters and arrangements of filter media are considered, by those with skill in the art, to be variously suitable for application to molten metal fluids, for purposes of filtration, as "single application" type filters. A single application filter is a filter which is placed into the stream of a flowing molten metal fluid from only a single heat of molten metal from the melting furnace. Once the single heat of molten metal is flowed through the filter medium, the filter medium is normally removed and discarded, whether the medium is clogged with impurities, thus ending its useful life, or whether it still has potential useful life when treated as described in the next paragraph. The discarded filter medium is then replaced by a new, unused filter medium. The entrapped impurities formed on or near that surface of such filter medium, which is directly exposed to the flow of molten metal, are coated with solidified and oxidized molten metal (the residue or the last remnants of the heat of molten metal which was flowed through the filter).
There is presently no means accepted by those with skill in the field to clean, renew, or rejuvenate these spent filter media to enable their reuse short of keeping the filter medium continuously heated or keeping it continuously submerged in molten metal, or reheating the filter medium prior to the introduction of succeeding batches of molten metal, all of which are costly and tedious procedures in application. Also, when a single application filter is kept in a reusable state, under any of the foregoing procedures, the filter is never removed from situ, due to substantial difficulties encountered in handling hot filters as described in Nowak, mentioned above. Therefore, the agglomeration of impurities accumulated on the surface of the filter, known as "filter cake", "sheds" or drops off between heats. The filter cake, left after the first heat of metal, which is considered to be almost totally composed of impurities, rapidly forms again on the exposed filter medium surface during the initial start-up of the succeeding heat. Thus the ability for succeeding uses of a single application filter is drastically reduced regardless of how the filter medium is kept in the ready state.
Needless to say that the cost of replacement of spent single application filter media is also relatively great compared to that of a filter which might be used multiple times without replacement or substantial loss of ability for re-use. Thus, it is believed that there is a commercial need to develop a porous ceramic molten metal filter which can be purchased for a cost generally equivalent to the single application filters presently being used but which could be used multiple times before discard without substantial loss of ability for re-use.